It is conventional to test blood donated to a blood bank by dispensing it from a pipette tip into wells of a microtiter plate for reaction to determine the presence of an infectious disease. It is critical in these tests that the correct amount of blood be dispensed, no more and no less. That in turn is dictated in part by the proper volume of blood being aspirated into the tip prior to dispensing. Such a pipette tip is provided with a conductive plunger that senses the level of liquid in the primary container. After aspiration, although it can detect liquid in the tip prior to withdrawing from the primary container, it cannot determine the condition of the liquid, that is, if it has bubbles. The reason is that air bubbles are surrounded by liquid and are still conductive.
Such a conventional system has worked well in most instances. There are isolated examples, however, when that system does not adequately detect improper dispensing:
a) if blood sample hangs from the tip as a pendant drop following aspiration or dispensing, or PA1 b) if air bubbles or the like have been aspirated into the tip so that the sample aspirated is less than expected because of the air bubble(s), or PA1 c) the liquid is forced above the plunger. PA1 a) aspirating the liquid into one of the tips to fill the tip to a nominal level; PA1 b) scanning the liquid in the tip by a scanner comprising a light source and a light detector, the source and detector being aligned along a direction that is generally normal to the tip axis; PA1 c) providing relative motion between the tip and the scanner while step b) is conducted, so that the scanner scans from a first position below the tip where a pendant drop of the aqueous liquid would be located if present, to a predetermined second position above the first position that represents the nominal level; PA1 d) while conducting steps b) and c), separately detecting at each of several portions of the tip between and including the first and second positions, the amount of light transmitted to the detector, so that all liquid-containing levels within the tip are detected; PA1 e) comparing the detected light of step d) with light known to be transmitted if liquid is present only at all of the levels, free of air bubbles; and PA1 f) flagging the results as being in error if water is detected at the first position, free of air bubbles, or if it is not detected at any one of the levels within the tip up to and including the second position; or if it is detected above the second position. PA1 a) a scanner comprising a light source, a light detector, and means for transmitting light from the source to the detector at a wavelength which is between about 630 and about 2000 nm; PA1 b) moving means for providing relative motion of one of the tips past the scanner so that the scanner scans the tip and its contents; PA1 c) a controller of the moving means and the detector that directs the detector to detect light transmitted through the tip to the detector at at least several adjacent portions of the tip, as well as just below the tip where any pendant drop would be located; and PA1 d) a comparator that compares the light transmitted at the several levels with that known to represent the tip containing bubble-free liquid at each of the levels.
Condition (a) is a problem following aspiration because the nominal volume is present in the tip, plus an additional volume is present pendant from the tip. That additional volume gets added to the nominal volume at the next dispensing, so as to alter significantly the volume to be tested. It must be noted that the problem of the pendant drop can occur after dispensing as well--if all or a portion of the dispensed volume perfuses up the exterior side of the tip during dispensing, and remains there as a pendant drop after dispensing, the volume intended to have been so dispensed into a well for testing will be short by the amount of the perfused drop(s). Condition a) can also be indicative of a clot having formed from fibrin, now in or on the tip, which can also cause air bubbles in the tip. Similarly, c) is caused by clotting so that the dispensing pressure forces liquid above the plunger.
Condition (b) is a problem because, contrary to the result detected by the conductive plunger, insufficient volume is actually present for dispensing, as represented by the volume of the air bubble(s).
Other solutions have been considered but are unsatisfactory. The use of a pressure transducer, to track tell-tale pressure changes during metering, is not feasible due to the presence of the conductive plunger within the tip. The use of a digital camera to record images of the tip for comparison of the raw data against a "visual standard" is expensive and imprecise. Visual inspection is obscured by the optical complexities of the tip. Lipemic, hemolytic or icteric samples can visually obscure bubbles.
Accordingly, there has been a significant need prior to this invention to improve the detection of improper aspiration/dispensing of blood samples for such things as blood bank testing.